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Langmuir 2008, 24, 9508-9514
Superhydrophobic Surfaces from Various Polypropylenes R. Rioboo,*,† M. Voué,† A. Vaillant,† D. Seveno,† J. Conti,† A. I. Bondar,‡ D. A. Ivanov,§ and J. De Coninck† Centre de Recherche en Mode´lisation Mole´culaire, UniVersite´ de Mons-Hainaut, Parc Initialis, AVenue Copernic, 1, B-7000 Mons, Belgium, Laboratoire de Physique des Polyme`res, UniVersite´ Libre de Bruxelles, CP233 BouleVard du Triomphe, B-1050 Bruxelles, Belgium, and Institut de Chimie des Surfaces et Interfaces, CNRS UPR 9069, BP 2488, 15 rue Jean Starcky, F-68057 Mulhouse Cedex, France ReceiVed April 24, 2008. ReVised Manuscript ReceiVed June 10, 2008 Superhydrophobic surfaces were prepared from solutions of isotactic polypropylenes of various molecular weights using soft chemistry. Varying the conditions of the experiments (polymer concentration and initial amount of the coated solution) allowed us to optimize the superhydrophobic behavior of the polymer film. Results show that decreasing the concentration and/or film thicknesses decreases the probability to get superhydrophobicity for all polypropylenes tested. Measurement and analysis of advancing and receding contact angles as well as estimation of surface homogeneity were performed. Similar results were obtained with syndio- as well as atactic polypropylenes.
1. Introduction For over a decade, the topic related to the super- or ultrahydrophobicity of solid surfaces has been attracting much attention. Numerous promising applications have stimulated the research mainly in three areas: enlarging the set of materials leading to superhydrophobic surfaces, determining the necessary and sufficient conditions to obtain this property, and facilitating the fabrication of such surfaces. Although the question concerning the necessary and sufficient condition to obtain a superhydrophobic system is still largely open, the underlying physical mechanism is generally accepted. Superhydrophobicity is due to the trapping of air below the drop,1 and the overall wettability is the consequence of composite wetting that follows the Cassie-Baxter rule:2
cos θSH ) φ1(cos θ0 + 1) - 1
(1)
where θSH and θ0 are the equilibrium static contact angles on the superhydrophobic surface and on a flat and smooth surface of the same composition, respectively. φ1 is the fraction of the solid/liquid interface in contact with the solid. The Cassie-Baxter equation, directly derived from the original Cassie equation, accounts for the wetting behavior of drops on top of a composite material made of solid and air. Practically, it is often used to describe the wettability regime of drops on structured surfaces, but, implicitely, the Cassie-Baxter equation assumes that the composite surface made of material and air, in contact with the liquid, is flat. For microscopically rough materials, Wenzel’s roughness r, which describes the ratio between the real and projected surface areas, has to be introduced in eq 1, as proposed, for example, by De Coninck and co-workers:3
cos θSH ) φ1(r cos θ0 + 1) - 1
(2)
The key point for the use of these equations is therefore the value of φ1. On superhydrophobic surfaces, the real contact area between liquid and solid materials is tremendously decreased. φ1 can be * Corresponding author. E-mail:
[email protected]. † Universite´ de Mons-Hainaut. ‡ Universite´ Libre de Bruxelles. § Institut de Chimie des Surfaces et Interfaces.
(1) Que´re´, D. Rep. Prog. Phys. 2005, 68, 2495–2532. (2) Cassie, A. B. D.; Baxter, S. Trans. Faraday Soc. 1944, 40, 546–551. (3) De Coninck, J.; Miracle-Sole´, S.; Ruiz, J. Phys. ReV. E 2002, 65, 036139.
either experimentally determined as in the work of Krupenkin and co-workers,4 where φ1 is measured around 6 × 10-3, or deduced from eq 1 as in the article of Hosono and co-workers,5 where the estimated value of φ1 is about 6 × 10-4. Many methods exist to produce such surfaces, among which mimicking natural leaves,6,7 casting of polymers8–10 and building roughness like a fakir carpet11,12 are a few typical examples.1,13–18 Beyond the agreement on the basic mechanisms, the details of this phenomenon are still a matter of debate, mostly because the experimental conditions required to prepare such surfaces are very diverse. Even the use of a hydrophobic material is not mandatory. Despite the Cassie-Baxter rule (eq 1), Hosono and co-workers5 experimentally showed that preparing superhydrophobic surfaces was possible with materials presenting equilibrium contact angles with water less than 90°.19 The requirement of having various (at least two) length scales is claimed by some authors,20–22 while others observed superhydrophobicity on nanoor submicron periodic patterned surfaces apparently presenting (4) Krupenkin, T. N.; Taylor, J. A.; Schneider, T. M.; Yang, S. Langmuir 2004, 20, 3824–3827. (5) Hosono, E.; Fujihara, S.; Honma, I.; Zhou, H. S. J. Am. Chem. Soc. 2005, 127, 13458–13459. (6) Barthlott, W.; Neinhuis, C. Planta 1997, 202, 1–8. (7) Furstner, R.; Barthlott, W.; Neinhuis, C.; Walzel, P. Langmuir 2005, 21, 956–961. (8) Shibuichi, S.; Onda, T.; Satoh, N.; Tsujii, K. J. Phys. Chem. 1996, 100, 19512–19517. (9) Onda, T.; Shibuichi, S.; Satoh, N.; Tsujii, K. Langmuir 1996, 12, 2125– 2127. (10) Erbil, H. Y.; Demirel, A. L.; Avci, Y.; Mert, O. Science 2003, 299, 1377– 1380. ¨ ner, D.; McCarthy, T. J. Langmuir 2000, 16, 7777–7782. (11) O (12) He, B.; Patankar, N. A.; Lee, J. Langmuir 2003, 19, 4999–5003. (13) Groenendijk, M.; Meijer, J. J. Laser Appl. 2006, 18, 227–235. (14) Callies, M.; Que´re´, D. Soft Matter 2005, 1, 55–61. (15) Li, X. M.; Reinhoudt, D.; Crego-Calama, M. Chem. Soc. ReV. 2007, 36, 1350–1368. (16) Ma, M. L.; Hill, R. M. Curr. Opin. Colloid Interface Sci. 2006, 11, 193– 202. (17) Blossey, R. Nat. Mater. 2003, 2, 301–306. (18) Nakajima, A.; Hashimoto, K.; Watanabe, T. Mon. Chem. 2001, 132, 31– 41. (19) Herminghaus, S. Europhys. Lett. 2000, 52, 165–170. (20) Gao, L. C.; McCarthy, T. J. Langmuir 2006, 22, 2966–2967. (21) Shirtcliffe, N. J.; McHale, G.; Newton, M. I.; Chabrol, G.; Perry, C. C. AdV. Mater. 2004, 16, 1929–1932. (22) Patankar, N. A. Langmuir 2004, 20, 8209–8213.
10.1021/la801283j CCC: $40.75 2008 American Chemical Society Published on Web 07/23/2008
Superhydrophobic Polypropylenes
¨ ner and McCarthy11 showed the only one length scale.7,11,12,23 O importance of the shape of the roughness elements on superhydrophobicity. According to Bico et al.,24 superhydrophobicity is also a function of the drop size compared with the size of the roughness elements. He et al.12 showed that, depending on the way the drop was deposited on the solid surface, sticking of the drop to the surface was possible. Falling drops resulted in the onset of a Wenzel regime25 instead of a Cassie-Baxter one3 when the drop was gently deposited.26,27 Besides the static wettability characteristics, the critical sliding angle R is also important. This angle is the angle of inclination of the solid surface from which the drop starts to move. As pointed out by some authors,11,16,28–33 characterizing the wettability of superhydrophobic surfaces requires one to systematically provide measurements of both the advancing and receding contact angles. In this way, contact angle hysteresis will be known, and the water repellency will be related to the critical sliding and static contact angles. Unfortunately, very often, only the one sessile drop contact angle is reported in the literature, preventing differentiation between the Wenzel and Cassie-Baxter regimes. Among the easiest and lowest-cost methods to prepare superhydrophobic surfaces on a large scale is that proposed by Erbil and co-workers10 for polymers. They cast solutions of isotactic polypropylene (iPP) in a good solvent (p-xylene) or in a mixture of good and poor solvents (p-xylene/2-butanone, for example) to prepare porous-like surfaces. The presence of a poor solvent results in a better homogeneity of the resulting surface and a higher static contact angle. The authors explain this phenomenon in terms of increase of nucleation rate, increase of polymer precipitation, and better initial spreading of the solution on the solid substrate. The behavior of a sole type of iPP (molecular weight, Mw ) 250 000) was investigated, with film thicknesses comprised between 10 and 100 µm and a concentration between 10 and 40 mg/mL. Resulting surfaces obtained from binary solutions were reported as inhomogeneous. The contact angle varied between 105° and 155° also depending on the evaporation temperature. No information on hysteresis was provided. Effects of film thickness and concentration on contact angles or superhydrophobicity were not given. In this article, we investigate in more details the conditions required to get the superhydrophobicity for such materials. Two coating methods were used to obtain a wide range of film thickness from simple binary solution of p-xylene and PP. The research focus is put on the effects of the concentration of polymer, of the thickness of deposited material and of the PP grade on the superhydrophobicity of the surfaces.
Experimental Methods (a) Materials. Commercial PP grades were provided by Total Petrochemicals (Belgium), Aldrich (Germany), and (23) Shiu, J. Y.; Kuo, C. W.; Chen, P. L.; Mou, C. Y. Chem. Mater. 2004, 16, 561–564. (24) Bico, J.; Thiele, U.; Que´re´, D. Colloid Surf., A 2002, 206, 41–46. (25) Wenzel, R. N. Ind. Eng. Chem. 1936, 28, 988–994. (26) Reyssat, M.; Pe´pin, A.; Marty, F.; Chen, Y.; Que´re´, D. Europhys. Lett. 2006, 74, 306–312. (27) Bartolo, D.; Bouamrirene, F.; Verneuil, E.; Buguin, A. Europhys. Lett. 2006, 74, 299–305. (28) Youngblood, J. P.; McCarthy, T. J. Macromolecules 1999, 32, 6800– 6806. (29) Yao, Y.; Dong, X.; Hong, S.; Ge, H.; Han, C. C. Macromol. Rapid Commun. 2006, 27, 1627–1631. ¨ ner, D.; Youngblood, J. P.; (30) Chen, W.; Fadeev, A. Y.; Hsieh, M. C.; O McCarthy, T. J. Langmuir 1999, 15, 3395–3399. (31) Dorrer, C.; Ru¨he, J. Langmuir 2007, 23, 3820–3824. (32) Miwa, M.; Nakajima, A.; Fujishima, A.; Hashimoto, K.; Watanabe, T. Langmuir 2000, 16, 5754–5760. (33) Que´re´, D.; Azzopardi, M. J.; Delattre, L. Langmuir 1998, 14, 2213–2216.
Langmuir, Vol. 24, No. 17, 2008 9509 Table 1. Physical Characteristics of the PP (Mn: Number Average Molecular Weight, Mw: Molecular Weight, Cryst: Percentage of Crystallinity) polymer
tacticity
PP1 PP2 PP3 PP4 PP5 PP6
isotactic isotactic isotactic isotactic atactic syndiotactic
a
Mn
Mw
% of cryst.a
5000 50 000 166 000 3700 54 000
12 000 190 000 580 000 14 000 127 000
31 30.6 28.8 3.2 16
From Lyons44 and Guadagno et al.45
Goodfellow (U.K.) (Table 1). 2-Butanone (Acros Organics, purity 99+%), p-xylene (Aldrich, 99+%), and isopropanol (ChemLab, 99.8+ purity) were used as received without further purification. Milli-Q water was used to test the wettability of the surfaces. Solutions of PP were prepared by dissolution of the polymer in p-xylene at 135 °C under reflux. Polymer concentration was varied between 1.66 mg/mL and 166 mg/mL. (b) Coating Methods. All coatings were performed on iPP sheets (Goodfellow, 1 mm-thick, PP303100/13), degreased with isopropanol. Two different methods (dip-coating and casting) were used in order to vary the final thickness of the film. By varying the thickness, one probes the relative importance of evaporation of the solvent and crystallization of PP. After complete dissolution of the polymer, the solutions were cooled by thermal inertia at ambient conditions to increase the solution viscosity and favor the deposition of PP. The coating of the polymer substrates was carried out when the temperature of the solution reached the range 55 ( 3 °C to 65 ( 3 °C and no gelation was visually observed. The dipcoating experiments were performed at controlled temperature because temperature influences the surface state of the resulting film.10 The surfaces were plunged in the solution at velocities varying between 1 and 3 cm/s and withdrawn after 1 s at the same speed. To bypass the control of the plunging and withdrawal speeds, the initial (i.e., wet) film thickness was deduced from weighing of the (dry) sample before and after coating and from the polymer concentration. The evaporation of the solvent lasted between a few minutes and a few tens of minutes at ambient temperature. Proceeding in such a way contributed to the reproducibility of the experiments and avoided large heterogeneities on the surface. The casting experiments were performed using open glass tubes (height 35 mm, inner diameter 22 mm) as vials to cast the solution and evaporate the solvents. These tubes were sealed on the iPP sheets using paraffin wax. The volume and concentration of the PP solutions were varied. The evaporation of xylene slowly took place at room temperature (21-23 °C) and atmospheric pressure. The relative humidity was in the range of 30-55%. The process lasted at least for 12 h, but one or two days were typically required, depending on the concentration and quantity of solvent to evaporate. (c) Wettability Measurements. Static as well as dynamic contact angles were measured using a Kru¨ss DSA100 contact angle analyzer using the sliding drop method.37 A drop of a given volume (∼15 µL) is brought to contact with the surface. Because of the high wettability contrast between the sample surface and the needle, the drop does not detach from it and forms a liquid bridge. The substrate is translated as slowly as possible (V ) 0.06 mm/s) over 5 mm (Figure 1A). The images are recorded at a frame rate of 12.5 images per second using a digital camera and stored for further analysis. After extraction of the contour of the drop, a spherical cap is automatically fitted
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observations were performed without metallization of the sample at the water pressure of 1.00 Torr and accelerating voltage of 30 kV. Additional measurements of roughness parameters were performed using a Wyko NT1100 optical profilometer (imaged surface: 119 µm × 91 µm, 736 pixels × 480 pixels).
3. Results and Discussion
Figure 1. (A) Schematic description of the sliding drop method. (B) Variation of the receding contact angles of a water drop as a function of the substrate displacement (V ) 0.060 mm/s). (drop volume: 15 µL) (I: homogeneous superhydrophobic PP1 surface, initial film thickness: 1 cm; II: heterogeneous superhydrophobic PP1 surface, initial film thickness: 1.5 cm). C ) 0.1 g/mL in both cases.
to it over a large region around the contact line with specially written software. The processing is automatically carried out on the recorded images providing hundreds of advancing and receding contact angles that permit statistical analysis. The drop size was always less than the capillary length, justifying the use of the spherical cap approximation. The test of Gao and McCarthy36 was also performed. This test consists of bringing (from the top) the superhydrophobic surface toward a drop. After the contact took place, the solid is separated from the drop. The quality of the superhydrophobicity is assessed from the presence or absence of a liquid bridge between the drop and the surface while removing the surface from the drop. The Johnson and Dettre method34 was used for the surfaces presenting no superhydrophobicity. A rapid testing method was setup to systematically determine the superhydrophobicity of the surfaces. This method, combining simplified versions of the sliding drop method37 and of the tilting angle method35 consisted of two steps. In the first one, a 15 µL drop is deposited on the surface, and the sliding angle is checked. The second step allows an attached drop to a needle to slide along the entire surface and checks the presence of the contact line pinning. The surface was considered superhydrophobic when no pinning was visualized and a low sliding angle (
